Page:Popular Science Monthly Volume 20.djvu/363

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window-panes. In other cases they unite in radial groups about certain centers, and thus build up globular masses to which the name of "spherulites" has been given.

In Figure 1 No. 1 represents a glass through which microliths or crystallites of different dimensions and character are diffused. In

PSM V20 D363 Sections of igneous rocks.jpg

Fig. 1.—Sections of Igneous Rocks, illustrating the Passage from the Glassy to the Crystalline Structure. 1. Vitreous rock. 2. Semi-vitreous rock. S. Vitreous rock with spherulites. 4. Rock with cryptocrystalline base. 5. Rock with microcrystalline base. 6. Rock of granitic structure built up entirely of crystals.

Nos. 2 and 3 the crystallites have united to form regular groups. No. 4 represents a glassy ground-mass containing microliths (a crypto-crystalline base), through which distinct crystals are distributed, and is typical of the features presented by most lavas. Nos. 5 and G illus-